Amenazas a la validez de la revisión de literatura

4.2. Introduction

Sediments that are deposited by or significantly affected by alongslope flowing bottom currents, know n as contourites, provide a valuable source o f inform ation regarding the activity o f bottom currents o f a given area/region during the past, w hich in turn can be related to changes in global clim ate (Stow et al., 2002). The recognition o f the significant influence o f therm ohaline bottom currents in the deep ocean basins (Heezen et al., 1966) in com bination w ith an explosion in quantity and quality o f offshore seismic and well data associated w ith increased petroleum exploration has allow ed significant advances in the interpretation o f contourite drifts, particularly their seismic characteristics (Faugeres et al.,

1999; Rebesco and Stow, 2001). One o f the m ost valuable characteristics o f contourites is that they hold a record o f past therm ohaline current activity, w hich in turn provides

inform ation about past clim atic conditions (Stow et al., 2002). Identifying records o f change in therm ohaline current activity and clim ate w ithin these sedim ents is often achieved via high resolution isotopic and sedim entological analysis o f core data, usually for specific events such as glacial-interglacial fluctuations or m assive iceberg discharge events (e.g.

Raymo et al., 1989; M cCave et al., 1995). This high resolution approach allows recognition o f current fluctuations to sub-m illennial tim e scales w hich is clearly o f im mense value in studies o f rapid clim ate change, but is often lim ited to the relatively recent past (<100ka, A lley and Clark, 1999; A lley et al., 1999). C ore-based studies are also inherently lim ited by their one-dim ensionality, w hereas geophysical techniques aimed at defining the internal architecture o f drift bodies have the advantage o f being able to dem arcate loci o f enhanced deposition or erosion in three dim ensions (e.g. K nutz and Cartwright, 2003, K nutz and Cartwright, 2004). Furtherm ore, the h igh er vertical resolution achieved at greater depths by m odem seismic data enables current distribution and strength to be inferred over long periods o f time (Ma, e.g. B erggren and H ollister, 1974; Davies et al., 2001; Uenzelm ann- Neben, 2002; Schut and U enzelm ann-N eben, 2005). H igh resolution three dimensional multi-channel seismic surveys, particularly in deepw ater slope environments, open still greater potential for recognition and analysis o f drift bodies, and raise the potential for the 3D analysis o f subtle architectural elem ents in order further constrain spatial and temporal variations in palaeocirculation.

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Chapter Four Late Neozene Contourites

One o f the m ajor challenges facing m odem palaeoceanography is to reconstruct past w ater mass geom etries and evolution, particularly in key areas o f ocean current flow such as

‘ocean gatew ays’. The present day Faeroe Shetland Basin forms a critical oceanic gateway for the exchange o f w ater m asses betw een the N orth Atlantic and the N orwegian G reenland Sea w hich in turn plays a cm cial role in the m ediation o f climatic conditions in western Europe (Berggren and Schnitker, 1983; Broecker and Denton, 1990; Rahmstorf, 2002).

Previous research suggests that the basin has acted as a key oceanic gateway throughout the late Neogene (K nutz and Cartwright, 2003, Stoker, 2003; Knutz and Cartwright, 2004). The Faeroe Shetland B asin is represented b y a deepw ater channel, the Faeroe Shetland Channel that separates the Scottish M assif from the Faeroe Platform (Fig. 4.1, Bott, 1984). The Channel forms the deepest conduit across the G reenland-Scotland Ridge, w hich in turn forms a barrier to exchange o f cold deep w ater m asses between the N orwegian-G reenland Sea and the N orth A tlantic (Nielsen, 1983; H ansen and 0 sterh us, 2000). The presence o f the Greenland Scotland R idge allows the developm ent o f a perm anent therm ohaline gradient between Nordic Seas and the N orth A tlantic that is balanced by a near geostrophic flow o f northerly sourced deep w aters through the Faeroe-Shetland Channel, which constitutes the second m ost volum etric ally significant gatew ay after the D enm ark Strait (1.7Sv Vs 2.9 Sv) (M iller and Tucholke, 1983; D ickson and Brow n, 1994; Hansen and 0sterhus, 2000). As a result o f this circulation, the w ater m ass structure w ithin the channel consists o f cold, dense deep waters form ed b y therm al densification o f surface waters in the N orwegian G reenland Sea flowing to the SW below c.400-600m , overlain by warm er surface waters form ing the NE return flow (Fig. 4.1, Turrell et al., 1999; H ansen and 0sterh u s, 2000; M asson, 2001).

The w arm surface w aters consist o f N orth A tlantic W ater and M odified N orth Atlantic Water, while the deep w ater is largely com posed o f N orw egian Sea O verflow W ater (Turrell et al., 1999; Hansen and Osterhus, 2000). The w ater m asses are distributed asym m etrically w ithin the basin, w ith southerly flow ing deep w aters restricted to the Faeroese slope b y the Coriolis Force before they enter the N orth A tlantic via the Faeroe Bank Channel. M easured deep w ater current velocities can regularly reach 60cm /s but on occasion exceed 100 cm/s in areas where Coriolis force focuses the current at the base o f the Faeroes Slope (Hansen and 0sterhus, 2000; M asson et al., 2005). The tim ing o f onset o f a m odem day style

therm ohaline circulation through the Faeroe Shetland Basin is thought to date from the m iddle Eocene, based on the identification o f the Eocene contourite drifts at the southern end

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Chapter Four Late Neozene Contourites

F a ero e Islan ds

‘500m

■Shetland Islands

[Iceland

Basin

Figure 4.1

Location map of the Faeroe Shetland Basin illustrating the key 3D and 2D data used in this study (3D: 1) and by Knutz and Cartwright (2004) (3D:2). Also indicated are the locations of seismic profiles (figure number circled, number refers to the location of seismic profiles within this Chapter, e.g. ‘Fig 3' refers to Figure 4.3.), the spatial distribution of Figures 4.10 (dotted line) and 4.11 (solid line), and the location of well 214/4-1. Yellow filled contours represent 500m, 250m and 50m thickness distribution o f WSD Slope section. Coloured arrows represent present day ocean currents: red arrows = surface inflow waters; blue arrows = SW flowing deep waters. Note location of Eocene slope progradation system and influence on WSD Slope thickness distribution. Abbreviations: 2DHR = high resolution 2D data; FBC = Faeroe Bank Channel; MKR = Munkagrunnar Ridge.

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